watchdog: sp5100_tco: Convert to use watchdog subsystem

Convert to watchdog subsystem. As part of that rework, use devm functions
where possible, and replace almost all static variables with a dynamically
allocated data structure.

Cc: Zoltán Böszörményi <zboszor@pr.hu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>